1. Field of the Invention
This invention relates to a method of manufacturing a liquid crystal display panel and a liquid crystal display panel manufacturing apparatus.
2. Description of the Related Art
Generally, a liquid crystal display panel is formed by superposing a pair of glass substrates with spacers interposed therebetween, bonding the substrates with a frame-shaped sealing member, and injecting liquid crystal into a space surrounded by the sealing member and the substrates. The spacers are arranged between the pair of substrates to keep a gap between the substrates uniform with a predetermined thickness. Thereby, a uniform display quality over the whole screen is secured. As the spacers, used are spherical spacers such as plastic beads scattered on a substrate, rod spacers such as glass fibers, or columnar spacers arranged in desirable parts on the substrate by photolithography in which photo-sensitive resin is applied on the substrate and subjected to exposure and development.
In a method of manufacturing a liquid crystal display panel having the above structure, two substrates in each of which transparent electrodes and alignment films are formed are bonded together, and thereafter liquid crystal is injected into the space between the substrates. A vacuum method is used as a general liquid crystal injecting method. In the vacuum method, first, an empty display panel with a liquid-crystal intake opened is arranged in a vacuum apparatus, and the display panel is evacuated. In this state, the liquid-crystal intake is immersed in a liquid crystal boat filled with liquid crystal, and then the pressure is gradually returned to atmospheric pressure to inject the liquid crystal into the display panel. Further, proposed is a method of shortening the injecting time by raising the temperature of the display panel during injection of liquid crystal under atmospheric pressure to lower viscosity of the liquid crystal. This method is called heating injection.
After the liquid crystal injecting step, the liquid-crystal intake is sealed. As disclosed in, for example, Jpn. Pat. appln. KOKAI publication NO. 2-87114, the sealing method is pressurizing sealing method in which the liquid-crystal intake is sealed in the state where the liquid-crystal-injected display panel is pressurized. Further, Example of the sealing method is heating sealing method in which the liquid-crystal intake is sealed in the state where the liquid-crystal-injected display panel is heated.
Usually, display panels are carried at room temperature (about 25° C.) from injecting step to sealing step. Therefore, when a liquid crystal display panel subjected to heating injection is put into the sealing step, the temperature of the liquid crystal display panel falls to about room temperature.
According to the liquid crystal display panel structured as described above, in environments of room temperature, the gap between the substrates is kept uniform by the spacers, and the thickness of the liquid crystal held in between the substrates is kept uniform in each part. Therefore, the liquid crystal display panel can perform good image display.
However, spacers and liquid crystal forming liquid crystal display panel expand in high-temperature environments. In particular, the expansion coefficient of liquid crystal is much greater than those of spacers and glass and the like. If the panel is used in high temperature environments, unevenness in display occurs due to the difference in the expansion coefficient.
Generally, the coefficient of linear expansion of columnar spacers containing pigments is about 0.8×10−4 [/° C.] or less, while the coefficient of linear expansion of liquid crystal is 2.5×10−4 [/° C.]. For example, supposing that the gap between the substrates of a liquid crystal display panel at normal temperature (room temperature) is 5.00 [μm], under a high-temperature of 50° C., the columnar spacers expand to have the height of 5.01 [μm], while the thickness of the thermal-expanded liquid crystal layer is 5.10 [μm].
Therefore, the thermal-expanded liquid crystal presses the substrates outward, and the substrates are deformed to bulge out. As a result, the spacers held between the substrates are separated from the substrates, and lose the function of maintaining the gap between the substrates. Further, the liquid crystal increased in volume by thermal expansion moves in the panel, and the space between the substrates is deformed to wave.
If the liquid crystal display panel in this state is vertically stood up, the liquid crystal increased in volume in the liquid crystal layer gathers in a lower portion of the liquid crystal display panel due to its own weight. As a result, the thickness of the liquid crystal layer increases in the lower portion of the liquid crystal display panel, and thereby unevenness in display occurs in image display and the display quality deteriorates.
To solve such a problem, in the sealing step, the liquid crystal intake is sealed after pressurizing the panel to discharge excess liquid crystal. In such a case, the liquid crystal is discharged from the narrow liquid crystal intake. Therefore, it takes at least 1 hour to discharge the excess liquid crystal, and all the discharged liquid crystal is discarded and wasted.
This invention has been made in view of the above points. The object of the present invention is to provide a method and an apparatus of manufacturing a liquid crystal display panel which can constantly maintain a good display quality without being influenced by the temperature of its use environment.